The ruling has put a spotlight on Australian efforts to intercept vessels on the high seas before they enter its waters and their occupants are entitled to claim refugee status. One boat has already been forced back to Sri Lanka after Australian officials ruled those aboard weren't eligible to claim asylum. The United Nations's refugee agency has criticized the policy of on-ship screening of refugees.

The Australian government yesterday pledged it would give 72 hours written notice before transferring the 153 asylum seekers to authorities there. It's unclear if this sets a precedent for future vessels intercepted at sea.

Human rights lawyers filed a challenge to the planned handover after the government confirmed on Sunday it had repatriated a separate group of 41 Sri Lankans intercepted in the Indian Ocean. A court in the city of Galle yesterday granted bail to 27 passengers. Five were remanded in custody and nine children were released. Anyone found guilty of leaving Sri Lanka illegally faces two years in prison.

Refugee lawyers said that Australia could be guilty of refoulement – the returning of refugees to countries where their lives or freedoms could be threatened – if screening was found to be inadequate. The government, which came to power last year after promising a tougher policy on illegal migration, argues that refugees intercepted outside Australian territorial waters aren't entitled to such protection.

After denying the existence of the boats for over a week, the government admitted Tuesday for the first time that 153 Sri Lankan asylum-seekers were in custody on an Australian Customs ship, without disclosing its location.

The court has adjourned until Thursday when government lawyers are expected to provide documents including details of the asylum seekers' names. Another hearing will take place within 21 days before the matter is taken up by the full bench of the High Court.

If the High Court upholds the challenge it would torpedo the government’s policy of turning back boats, one of the cornerstones of its border protection policy. But the court could take weeks to reach a decision, leaving the 153 asylum seekers in limbo.

"As far as we're aware, this is the first time Australian courts have had to consider a situation where a government is actively handing over people at risk to their persecutors," George Newhouse, a lawyer for the asylum seekers, told ABC Radio.

Video conference screening

Australian Human Rights Commission Chief Gillian Triggs said the screening of the 41 asylum seekers who were returned to Sri Lanka reportedly involved a four-question interview via video link. The applicants were not given an opportunity to challenge the screening process.

"It sounds as though three or four or five questions are being asked by video conference, snap judgements are being made, and they're simply being returned," Ms. Triggs told ABC television. "There is an obligation with international law to have a proper process."

One of asylum seekers returned to Sri Lanka, Dhamith Caldera, denied he had been screened as a possible asylum seeker. "They never asked any questions. They just wanted us to go back," he told Agence France-Presse.

The United Nations High Commissioner for Refugees today questioned the legality of the screening, saying its experience of on-sea processing had generally not been positive and expressing deep concern about the transfer of the 41 asylum-seekers.

“Such an environment would rarely afford an appropriate venue for a fair procedure,’’ the commissioner, Antonio Guterres, said in a statement.

The Australian Government insists it is honoring its international obligations. The Solicitor-General Justin Gleeson told the High Court the boat, whose passengers were now in Australian custody, was found outside Australia's migration zone, meaning that those on board had no right to claim asylum.

Opposition hails transparency

Australia's opposition has called the court’s interim ruling a victory for transparency. "What the High Court is now exposing is the extent to which the government has been able to hide from the Parliament and the Australian people its actions in the name of national security," Labor Sen. Kim Carr told Parliament yesterday.

Prime Minister Tony Abbott swept to power last September promising to adopt a hard-line policy against boat people. Asylum seekers arriving in Australia by boat are sent to offshore detention centres in the impoverished South Pacific nations of Papua New Guinea and Nauru with no prospect of settling in Australia, though they can apply to the UN for resettlement in a third country.

About 16,000 asylum seekers came on 220 boats to Australia in the first seven months of last year, when the previous government was in power. Mr. Abbott's government says there have been no illegal boat arrivals since December 2013.

A majority of Australians support the tougher asylum policy, including sending boats back when safe to do so, according to a recent poll by the Lowy Institute think tank.

Abbott last week described Sri Lanka as a "society at peace," drawing a sharp response from members of the Tamil community in Australia, which said human rights abuses have continued even after the civil war ended in 2009.

Tamils aboard

Not all the Sri Lankans seeking to enter Australian waters are Tamils; some are from the majority Sinhalese population. That may bolster the Australian government's claim that they are primarily economic migrants, rather than members of an oppressed minority. Most of the 153 people on board the intercepted boat are Tamils.

"If the Australian government wants to address the loss of asylum seekers’ lives, it should not be returning them to a country where their lives may be in grave danger," Amnesty International Australia’s refugee spokesperson Graeme McGregor said in a statement. "Australia stands alone in failing to recognise the ongoing human rights violations taking place in Sri Lanka."

Australia’s immigration minister, Scott Morrison, is due to visit Sri Lanka later this week to hold talks with government and defense officials, and will attend a ceremony with President Mahinda Rajapakse to mark Australia's gift of two former patrol vessels.